a) Field of the Invention
The invention relates to a device for cooling diode lasers, having channels which are arranged in superimposed planes and through which a cooling liquid flows.
The device is suitable as a heat sink for diode lasers, in particular for cooling diode laser arrays and stacks thereof.
b) Description of the Related Art
It is known that the use of high-power diode lasers always requires cooling, which can be achieved particularly efficiently by fluid-dynamic micro-channel cooling using water as the cooling liquid. Thus, considerable surface areas for the introduction of heat are achieved with so-called microchannel heat sinks in which the microchannels are made in a material with a good thermal conductivity using various methods.
The multiplicity of known microchannel heat sinks comprises different functional planes in a sequence of assembled, structured layers, the object which is to be cooled being arranged on an upper covering layer, for example by soldering. In addition to the covering, the layer structure also includes the functions of supplying and discharging the cooling liquid and the actual cooling via the microchannels.
For example, in a microchannel heat sink of the type described in DE 43 15 580, these functions are distributed over five layers. In a microchannel or distributor plate, the cooling liquid supplied via an inlet is distributed to the microchannels, which are situated beneath the diode laser attached to the covering layer. The cooling liquid is passed into a collecting plate via collecting channels in an intermediate layer, and from this collecting plate there is a connection to an outlet. A baseplate closes off the microchannel heat sink at the bottom. The modular structure is in principle suitable for vertical stacking.
Stackable systems are also described in U.S. Pat. Nos. 5,105,429 and 5,105,430, in which the cooling liquid is passed through the stack in continuous paths. Each of the microchannel heat sinks present in the stack comprises a multilayer structure having microchannels in the upper layer and has inlets and outlets which are connected to the continuous paths. In U.S. Pat. No. 5,105,430, the coolant does not flow parallel to the direction in which light is emitted from the installed laser array, but rather transversely to this direction, in the width direction of the microchannel heat sink. The cooling liquid is supplied to the center of the microchannels and leaves them in two directions. A drawback is the low flow velocity which occurs in the microchannels since the cross section of the microchannels is larger than that of the inlet, on account of the microchannels being connected in parallel.
With the multilayer distribution described in DE 197 50 879, the microchannels which are relevant for cooling are made in an upper and a lower layer and are connected by channels in a separating layer in such a manner that the cooling liquid initially passes through the microchannels in one plane and then through the microchannels in the other plane. In this case too, a drawback is the low flow velocity in the channels, since the flow cross section of the channel""s structure over branched groove patterns and taking the separating layer into account is greater than the cross section of the inlets or outlets.
Working on the basis of DE 43 15 580, a solution in accordance with DE 197 10 716 was based on the object of considerably reducing the pressure loss which the cooling liquid undergoes as it passes through the microchannel heat sink. For this purpose, the intermediate plate is modified by a layered arrangement in such a manner that the cross section of flow is successively adapted by means of a stepped structure.
The increase in the overall height which is caused by the layered arrangement has a particularly adverse effect for applications in which it is aimed to achieve large optical area power densities by means of a stacked arrangement of the diode lasers.
To summarize, it has to be stated that the measures aimed at improving the quality of microchannel heat sinks are not sufficiently satisfactory. In particular, they have adverse effects on stacks of heat sinks of this nature which are operated in parallel in terms of flow and in which an increase in the flow rate which is proportional to the number of heat sinks increases the pressure losses to an unacceptable extent.
Therefore, the invention is based on the primary object of increasing the heat transfer coefficient, with a low overall height of the device, in such a manner that the pressure losses occurring also effectively ensure that stacked heat sinks are operated in parallel in terms of flow. In doing so, it should be taken into account that the thermal resistance and pressure losses, on account of their effect on the flow velocity of the cooling liquid, cannot be optimized independently of one another.
According to the invention, the object is solved by a device for cooling diode lasers, having channels which are arranged in superimposed planes and through which a cooling liquid flows, in that the channels in each plane are divided into groups which are flow-connected in series with one another and, in order to be connected in series, open out into flow-connecting links which are common to the superimposed planes. A first group of the groups of channels is in communication with a common inlet and another group of the group of channels, which is the last group in the series, is in communication with a common outlet for the cooling liquid.
With the aid of the inventive measures, outside the attachment area of the object to be cooled, to avoid pressure losses, the largest possible cross section of flow is used, the relatively large area available for the introduction of heat here counteracting a reduction in the heat transfer coefficient caused by a reduced flow velocity. By contrast, in the region of the flow-connecting links which serve as a collecting pool for a plurality of channels, an increased flow velocity is generated, and the partial pressure loss which occurs only in a selectable region can be accepted on account of the improved heat transfer.
Further positive effects arise as a result of the fact that the groups of channels which are flow-connected in series in one plane have the same portion of coolant flowing successively through them. Unlike when channels lying in different layers are connected in series (DE 197 50 879), it is possible to reduce the flow cross section while maintaining a constant number of channels which are approximately equivalent in terms of their cooling effect. It is thus possible, for example, to reduce the overall flow through the heat sink and the flow velocity at the inlets and outlets without an associated reduction in the flow velocity in the channels, which has a particularly beneficial effect when it is necessary to stack microchannel heat sinks. On the other hand, if the flow is kept constant, it is possible to increase the flow velocity and to improve the cooling action.
The arrangement of the groups of channels in the superimposed planes increases the area available for the introduction of heat and has a beneficial effect on increasing the flow velocity for the cooling liquid. Advantageously, the channels which are arranged in superimposed planes are formed in layers, between which there is a separating layer having the common flow-connecting links.
Since the flow-connecting links which have a common action for the planes may act as additional channels which are relevant for cooling, it is advantageous if they are arranged in the region where the principle introduction of heat occurs, beneath the object which is to be cooled.
To increase the height of the channels, the layers may be designed as multiple layers.
The channels which are relevant for cooling may be aligned in various ways. For example, it is possible for the groups of channels in the superimposed planes to be designed either for flows of coolant which are oriented parallel to one another or, at least in parts, for flows of coolant which are oriented perpendicular to one another. The first variant can be varied still further in that the flows of coolant may run parallel to the direction of emission from the diode laser or perpendicular to this direction. In the second variant, one of the flows of coolant should run in the direction of emission from the diode laser.
For functional reasons, the layers which contain the groups of channels are covered by covering layers. To achieve an improved distribution of heat, it is advantageous if an upper covering layer is provided with a step in a region for attachment of the diode laser or if the upper covering layer bears an additional layer for attachment of the diode laser. The additional layer may enflush with the upper covering layer or may also form a step, at which the covering layer projects beneath the additional layer.
The invention is to be explained in more detail below with reference to the diagrammatic drawings.